Burner membranes containing sintered metal fiber webs are known from European patent application 0157432. The metal fibers used in accordance with this application are resistant to high temperatures.
Repeated use of these membranes causes the radiant sides of the surface layers to be subjected to very strong temperature fluctuations that vary from room temperature to possibly 1000.degree. C. These surface zones are thereby alternately subjected to strong thermal expansions and contractions. Irregularities in the porosity of that surface result in local temperature differences and therefore in mechanical stresses. The zones with the lowest porosity heat up the most. In the course of time (i.e. after having been subjected to a considerable number of cold/hot temperature cycles), this can occasion the formation of small checks (fissures), cracks or craters in the membrane surface.
Porosity increases at these cracks so that preferential channels are formed for fuel flow. This causes the formation of a blue flame, which must be avoided in the case of radiant burners (because a blue flame results in higher NO.sub.x emission). Besides, the blue flame formed has the tendency to further extend the crater or crack zone. Indeed, the very high flame temperature attacks the small crater walls further and attack deeper under the membrane surface (in the opposite direction of the gas supply), for instance by locally melting together the crater edge fibers there.
It is now the object of the invention to avoid these drawbacks and to counter degeneration, i.e. the formation of small craters or cracks during the use of the membrane.
In particular, it is the object of the invention to avoid these drawbacks in the case of radiant membranes the porosity of which is not completely uniform over their surface and/or through the thickness of their surface layer.
It is therefore an object of the invention to provide burner membranes for radiant burners, which membranes comprise, at least near their radiant surface, porous sintered fiber webs of inorganic fibers that are resistant to high temperature and with an enhanced resistance to degeneration due to temperature fluctuations, i.e. with a higher durability.
It is a further object of the invention to provide radiant burner membranes of sintered fiber webs which, despite a maybe less uniform porosity near their radiant surface, show a strongly reduced tendency to form blue flames, particularly after a longer time of use.
It is also the object of the invention to provide burner membranes whereby the extension of any small craters formed is strongly contained during further use, so that a further degeneration is stopped.
It is yet another object of the invention to provide burner membranes with a higher, more uniform and more durable heat radiation power and lower NO.sub.x emission, by containing crater formation and blue-flame formation.
Yet a further object of the invention deals with the provision of a radiant surface combustion burner comprising a housing with inlet means for the fuel supply and a burner membrane as herein further described at its outlet combustion side.
Finally it is an object of the invention to provide a process for radiant heating articles with increased efficiency, whereby the articles are disposed in front of the radiation side of a burner membrane according to the invention.
In particular, it is the object of the invention to provide sintered fiber-web membranes with a reduced tendency to degenerate, which have an average porosity of from 70 to 90% and preferably of from 77 to 85%. Moreover, the variation in permeability P (as defined hereinafter) from one place to another over the sintered sheet will preferably be lower than 25% and most preferably even lower than 10%. These membranes may be made in a flat, bent or cylindrical shape, as desired.
These objects are met in accordance with the invention by making grooves in the shape of a grid, at least into the membrane surface opposite from the fuel supply side: i.e. the surface at the radiant side. This precludes an uncontrolled formation and extension of these local cracks, if any, over the surface. Indeed, the grooves constitute barriers to the further proliferation of crack formation. Moreover, the grooves divide the surface into a kind of small waffles that can expand (and contract) in random directions parallel to the membrane surface, the small grooves growing narrower as temperature increases, or wider as the membrane cools down. Consequently, the temperature cycles then cause less local mechanical stresses in the membrane surface. So, the risk that cracks will be formed in the course of time is strongly reduced.
A sintered fiber membrane sheet in accordance with the invention generally has a thickness of about 2 to 5 mm. It is only an approximately 1 mm thick boundary layer on the radiant side which heats up strongly during burning. Therefore, it will be sufficient and it is indicated to make the grooves not deeper nor wider than 1.5 mm and preferably even less deep and narrower than 1 mm. Groove depths of between 7 and 15% of the total sheet thickness, e.g. about 10%, will be preferred.
On account of the intended uniformity, the groove grid preferably has meshes of nearly equal surface area. Preferably, the meshes are equal regular polygons such as equilateral triangles, squares, rhombi or regular hexagons. Their surface area is chosen between 4 mm.sup.2 and 400 mm.sup.2. Meshes that are smaller than 4 mm.sup.2 reduce the useful burner surface too much whereas there are too few barriers against crater proliferation if the meshes are larger than 400 mm.sup.2. Preferably, the mesh area is between 9 mm.sup.2 and 250 mm.sup.2 and most preferably between 20 mm.sup.2 and 150 mm.sup.2.